The invention refers to lighting panels for use in lighting units having fluorescent tubes as a lighting source, and more particularly refers to low luminance lighting panels of the type described in my above mentioned patents and which include an outer or light-emissive surface having a plurality of refracting elements in the form of cones or pyramids, the lateral surfaces of which are constructed such that luminous rays emerging from the cones or pyramids form angles of less than 60.degree. with the axis of symmetry of the cones or pyramids so as to obtain a lighting panel of theoretically nil luminance.
In my U.S. Pat. No. 3,716,709 a panel includes cones the cross section of which through an axial plane is defined by two lines symmetric with respect to the axis of symmetry of domes, each one of which constitutes a first arc the center of which is situated on the opposite side of the axis of symmetry and above the plane of the base of the dome, a straight line segment tangential to such first arc, and a second arc tangential to such straight line segment, the center of which is situated at the other side of the axis of symmetry of the dome and in the plane of the base of the dome.
The special structure of such refracting domes, in theory, cause all the rays which emerge from the domes to form angles of not more than 60.degree. with respect to the axis of symmetry of the domes. The remaining rays undergo total internal reflection and are reflected towards the interior face of the panel, thus being eliminated and giving a panel of theoretically nil luminance. However, in practice, some luminous rays with points of incidence at certain points on the interior or upper surface of the panel may emerge from the exterior face of the panel forming angles greater than 60.degree. with respect to the axis of symmetry of the domes. These rays which form an angle of between 60.degree. and 90.degree. with the axis of symmetry of the domes are particularly bright and therefore are of primary concern with respect to the luminance of the panel where the rays strike the interior face of the panel in a direction which is normal or substantially normal to the surface of the panel, since such rays undergo little energy loss in their journey through the panel.
In my U.S. Pat. No. 3,794,829 a low luminance panel is described having on its exterior surface a plurality of prismatic refracting elements in the form of domes having a curved lateral surface and rounded apex in which the luminance caused by rays striking the panel at points on the interior or upper face in a normal or substantially normal direction to the interior surface of the panel is eliminated.
The domes of the panel of such patent have circular bases and a peripheral surface defined by a line composed of first and second arcs the center of which are situated on opposite sides of the axis of symmetry of the dome and above the plane of the base of the dome, first and second straight line segments tangential respectively to the first and second arcs, and a third arc tangential to the straight line segments and having its center situated at the intersection of the axis of symmetry of the dome with the plane of the base of the dome.
The domes of the panel of the patent are positioned with the bases of each two adjacent domes superimposed on each other in such a way that the lateral surfaces of the domes intersect, thus reducing or eliminating flat portions of the panel between domes, and therefore reducing the luminance of the panel, the domes being positioned either in staggered relation, the intersected bases of the cones thus having hexagonal form, or arranged in orthogonal rows and columns, the intersected bases of the domes thus being in square form.
It has however been found that some of those luminous rays which have their points of incidence on the interior face of the panel at positions corresponding with the lines of intersection of the bases of the domes, or which have points of incidence close to these positions, are refracted to emerge from the external surface of the domes at angles of 70.degree. or more to the axis of symmetry of the domes, thus giving rise to a certain degree of luminance, while the other rays which have points of incidence at such mentioned positions on the interior surface of the panel undergo a series of total internal reflections. This leads to loss or to passing through the interior face of the panel, returning to the troffer of the apparatus with a large loss of luminous power, thus resulting in a lowering of the efficiency of the apparatus.